The control of single coloured light sources and the tuning of the light atmosphere inside a room created by several light sources require a proper measurement of illumination and colour patterns. This becomes especially important in the case of light emitting diodes (LEDs) as they tend to change their output spectra over time. Moreover, their output spectra are a function of the drive level and differ from one LED to the other. Consequently, correct control requires an accurate measure of the light output spectrum. Light sensors based on an array of photo-detectors with narrow band colour filters enable measuring the output spectrum. Every photo-detector measures a small part of the spectrum enabling the reconstruction of the entire spectrum.
Interference filters, for example based on a stack of dielectric layers, and so called Fabry-Perot etalons constitute the most common type of narrow band colour filters. Advantageously, these filters can have a very narrow spectral response. The central wavelength of the filter, however, depends strongly on the angle of incidence of the light. Consequently, the spectral response of a light sensor will differ for light rays impinging on it from different directions. This clearly constitutes a drawback, preventing an accurate measurement of the light spectrum. In addition, the narrow band filters transmit only a small amount of the spectrum, diminishing the sensitivity of the photo-sensor. Moreover, determining the spectrum with a high resolution requires a large amount of these filters.